Crack Paths 2012

surface, where the material does not exhibit any visible traces of an irreversible slip

activity in the closest vicinity of the crack.

Figure 15. Slip bands after 9.5 x 105 cycles.

Figure 16. The same area as in Fig. 15

after 6.2 x 106 cycles.

Figure 17. Tip of the fatigue crack shown in Fig. 12 (left upper corner).

Figure 19. Section through

Figure 18. Tip of the fatigue c ack shown in Fig. 12 ( ight bottom corner).

fatigue crack in its central

part.

The fatigue cracks develop from long slip bands. Evolution of long bans during high

cycle fatigue is documented in Figs. 15 and 16. The first one shows a region with slip

bands, which were formed during the first 9.5 x 105 cycles. The second Figure displays

the same area after 6.2 x 106 cycles. The length and the width of the slip bands have

increased. However, new bands appeared in areas, which were free of bands after

9.5 x 105 cycles, Fig. 15. There is a tendency for development of slip bands tilted at the

same angle to the loading axis and concatenating along one line. Successive

development of new bands and a moderate growth of old ones in length finally results in

creation of long slip bands. Indeed, the condition of near-by oriented zone of grains in

this region has to be fulfilled.

Fig. 12 indicates that the fatigue crack on both ends exhibits fuzzy crack paths,

contrary to its middle part. Both the crack tips are displayed in detail in Figs. 17 and 18.

Secondary slip bands are visible in the crack wake and near the crack tips. The slip

bands in the right upper corner in Fig. 18 were produced by the fatigue loading of the

bulk material; the slip bands near the crack tip are a result of the cyclic stress

concentration at the tip of the propagating crack.

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